The present invention relates to a catalyst which is used for wet oxidation treatment for wastewater containing organic compounds etc. and used for decomposing them and also, relates to a process for producing the catalyst and a process for treating wastewater with the wet oxidation under the presence of the catalyst.
There have been hitherto known methods for treating wastewater, such as a biochemical method called the activated-sludge method, a wet oxidation treatment called the Zimmerman method.
The activated-sludge method requires a long time to decompose organic compounds and also, requires to dilute wastewater up to a concentration suitable for the growth of algae and bacteria, so that it has a defect that a large scale of area is required to set treating facilities.
The Zimmerman method comprises treating wastewater in the presence of an oxygen gas under a high temperature and high pressure and decomposing organic compounds in the wastewater. In this method, there has been proposed a method which comprises a use of various kinds of oxidation catalysts in order to accelerate a reaction rate. The oxidation catalyst used here is a catalyst with a compound of a noble metal such as palladium, platinum or the like supported on a carrier such as alumina, silica, silica gel, active carbon or the like.
In general, it is rare that the chemical species included in wastewater to be treated is always the same. Thus, there are many cases where a nitrogen-containing compound is included besides a nitrogen-free organic compound.
However, wastewater including a nitrogen-containing compound such as an amine compound, an amide compound, an amino acid compound or the like is not treated with satisfactory efficiency by the above-mentioned methods.
Wastewater including an amine is usually treated by a cohesive treating method in which an anionic macromolecule cohesive agent is used. This method comprises gathering an amine by the anionic macromolecule cohesive agent and eliminating a formed precipitate (or sludge) from wastewater. Also, there has been attempted an adsorbent method which comprises bringing wastewater in contact with various kinds of adsorbents such as active carbon, activated clay, a silica gel, a complex oxide gel and the like, and adsorbing the amine to the adsorbent for eliminating it from the wastewater.
Since sludge formed by the cohesive treating method contains amines, it should not be abolished without a following treatment. Because of this, a treatment to decompose the amines in the sludge becomes necessary. In addition, the macromolecular cohesive agent is expensive, so a cost for the treatment becomes high.
Concerning the adsorbent method, the elimination percentage of amines is not satisfactory enough. Since the adsorbing power of an adsorbent easily decreases, there is a problem in durability of the adsorbent.
Since the wet oxidation process is unavoidable in the wastewater treatment, it will be convenient if an arrangement is made so as to decompose a nitrogen-containing compound included in the wastewater in the course of the wet oxidation process.
On the other hand, a treatment for wastewater including a sulfur-containing compound has been so far carried out by a respectively different method depending upon the state and nature of the sulfur-containing compound. For example, in a case of wastewater including an organic sulfur compound, a biological treatment is generally carried out. However, in a case where a compound containing thiophene and the like is treated, which affects badly upon organisms in sludge, a biological treatment can not be applied and, accordingly, a combustion treatment etc. are carried out.
Wastewater containing a sulfide such as sodium sulfide or the like: for example, wood digestion wastewater in paper- and pulp manufacture, wastewater coming out from a coke oven in steel manufacture, wastewater after fiber-washing, wastewater from a plant of petroleum chemical products such as ethylene, BTX and the like, as well as wastewater from a coal gasification plant, a petroleum-refining plant, a rayon factory and a dyeing plant; has been mostly treated by a method which comprises adding iron chloride into wastewater to solidify sulfur ions, removing solid iron sulfide by solid-liquid separation, adjusting pH of the separated solution, carrying out a biological treatment of the solution, and then discharging the treated wastewater. Also, wastewater containing a sulfite salt and thiosulfate salt: for example, wastewater coming out from a wood kiln of pulp-making factory, wastewater from photograph-developing, wastewater from metal treatment as well as alkaline wastewater used to absorb sulfur dioxide and the like; is treated by a method which comprises subjecting wastewater to neutralization-precipitation treatment followed by biological treatment and then discharging the treated wastewater.
When wastewater containing a sulfur-containing compound is treated by either one or both of biological treatment and combustion treatment, there exist the undermentioned problems to be solved. In the biological treatment, it is necessary to adjust a wastewater source solution by diluting it with water so that organisms are not badly affected. Therefore, wastewater to be treated becomes a large amount and facilities for the biological treatment must be arranged in a large scale, so that there is a serious problem is in the necessary cost and so forth.
Also, in the combustion treatment, when a heat amount generating from wastewater is low, a supplementary fuel must be added and also, because a large amount of sulfur is usually included in the wastewater, a large amount of sulfur oxides are formed and, therefore, it is necessary to arrange a desulfurizer.
Next, when wastewater including a sulfur-containing compound such as sulfide is treated, if a method which comprises removing the sulfur-containing compound as iron sulfide by adding iron chloride is applied, sludge having iron sulfide as a main component is formed and also, this method is complicate as a treating procedure, because it consists of the following steps: injection of solution of chemicals, solid and liquid separation, pH control and biological treatment.
The organic halogeno compounds have been used for various kinds of usage because of their stability. Since they are nonflammable and has great capability to degrease, they have been used in a large amount as a degreasing cleaner in metal, machinery and electronics industries as well as a cleaner for dry cleaning. On the other hand, the compounds have brought about problems on various fields. In general, since the organic halogeno compounds are difficult in decomposition, they are seriously accumulating in the natural environment and, as a result, ground water pollution has emerged everywhere. Furthermore, some of the organic halogeno compounds have been found to have carcinogenic nature against human bodies and, thus, trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane and the like have been designated as regulation items of the water-pollution preventive law on 1989, because of concern about influence on the human health.
Concerning treatment of the organic halogeno compounds, various methods have been proposed or used, and if the methods are roughly classified, there are a degradation method and a nondegradation method. Concerning the degradation method, there are listed a packed tower stripping method, a volatilizing method by means of exposing to air or heating, and an adsorption method using active carbon or macromolecules. Concerning the volatilizing method, the operation itself is very simple and at a low cost, but the method comprises only evaporating organic halogeno compounds in a liquid phase or a solution and scattering them in air and, therefore, basically it does not settle environmental pollution caused by organic halogeno compounds. Concerning the adsorption method, a secondary processing such as a recovering process after adsorption and a process to treat an adsorbent becomes necessary.
Concerning the degradation method, there are listed an irradiation method, a microorganism degradation method, a redox method and so forth. The irradiation method, of which representative examples are a photodecomposition method using a semiconductor as a catalyst and a radiation-irradiating method using a radiation, is still on an experimental stage and can not be adopted for a practical use. The microorganism degradation method takes a long time for treatment and its efficiency in treatment is unstable and, therefore, there exist many problems for a practical use. Concerning the redox method, a method of using an oxidizing agent such as ozone, hydrogen peroxide or the like and a method of reductive degradation method using iron have been attempted.
However, in a case where an organic halogeno compound exists in a high concentration, a method of highly efficient treatment has not yet been invented either as a nondegradation method or as a degradation method. In the volatilizing method, a large amount of organic halogeno compounds are discharged into air and, therefore, the method is not fundamental solution for the organic halogeno compounds to be treated. The adsorption method is short in the break-through time in a case of high concentration, so it is not practical. Concerning the degradation method, highly effective decomposition has not yet been a practical one, and also, there exists a problem that harmful decomposition products are secondarily generated. In short, a practical and fundamental method to remove the organic halogeno compounds is not yet developed at a present stage.